174 Mr. W. Spottiswoode. On Effects produced by [Feb. 19, 



from time to time at moderate velocity between the terminals. Erom 

 the known period of the machine, and the number of the discharges 

 crossed by these flashes in their passage from terminal to terminal, it 

 was calculated that the time of passage was about '03 of a second. 

 Occasionally there was a still brighter flash or meteor, which similarly 

 traversed the field, but with a velocity apparently of about double that 

 of the others. 



When the discharge was set horizontally, similar phenomena were 

 seen ; but at a distance about midway between the two terminals, the 

 flames appeared to meet, and at their point of meeting they showed 

 continuous luminosity throughout. When the flame was blown aside, 

 there appeared occasionally, and at rather long intervals, the true 

 spark, evinced by an irregular bright line,, reaching from one terminal 

 to the other. 



On observing the discharge in air attentively, it was noticed that 

 whenever a true spark passed vertically its passage was marked, as 

 usual, by an irregular bright line when its path was outside the 

 aureola or flame, but by a similar dark line when its path was within 

 the aureola. 



With reference to this phenomenon, it may be observed, in the first 

 place, that in every coil discharge the true spark is always anterior to 

 the aureola, which, in fact, immediately follows it. The true spark of 

 each discharge taken by itself will, therefore, always appear as a bright 

 line. It, however, sometimes happens, as mentioned above, that in the 

 succession of discharges produced by the present method, the flame or 

 aureola of one discharge lasts, at least over part of its length, until the 

 next takes place. When this is the case, the spark of the succeeding 

 discharge will pass partly through air, as usual, and partly through 

 the flame which has lasted over from the preceding discharge. This 

 part of its path will be through a conductor, which may be so far 

 better than the air, as to allow the spark to pass in a non-luminous 

 condition. The phenomenon noticed by Mr. De La Rue, viz., the 

 sudden expansion of a gas at the instant of the passage of an electric 

 discharge through it, may then take place, and the displacement of 

 the flaming gas over that part of the path may give rise to the appear- 

 ance of the dark line within the area of the aureola. The remaining 

 part of the path, if any, will be bright. It is, however, further to be 

 noticed that the true spark, or those portions of it which are bright, 

 always appears to lie outside, and never within, the aureola. This 

 may arise from the fact that on the extinction of a flame the portions 

 of space immediately outside are for the moment the hottest and most 

 conductive, and that the next spark will consequently choose a path 

 really outside the contour of the flame just extinguished. And as the 

 successive flames do not differ materially in size or shape, the bright 

 spark will appear always to lie outside the flame. 



